The present invention relates to liquid crystal compositions for liquid crystal display devices and to liquid crystal display devices using the compositions. More particularly, it relates to nematic liquid crystal compositions for liquid crystal display devices of supertwisted nematic (STN) mode and to liquid crystal display devices using the compositions.
Compact and, thin display devices exhibiting little electric power consumption and yet having similar display capacity and quality to those of conventional cathode ray tube (CRT) are required with the latest development of information instruments, particularly personal terminal equipments. For this reason, a liquid crystal display method using supertwisted nematic (STN) mode is widely adopted at present. The mode was proposed by T. J. Scheffer et al in 1984, which was epochmaking because the mode exceeded the limit of twisted nematic (TN) mode used for conventional liquid crystal displays. Namely, they reported that better contrast and broader viewing angles than those of the TN mode having a twist angle of 90 degrees were obtained by establishing the twist angle at 180-270 degrees to use birefringence effect of a liquid crystal cell, and that, even if the duty ratio was decreased (.about.1/480), the display quality was not so lowered. (For example, refer to T. J. Scheffer et al: Appl. Phys. Lett., 45 (1984) 1021.)
However, though the STN mode has distinctive features of a distinct contrast and a broad viewing angle, there remain problems of low production yield due to abnormal lighting of liquid crystal cells caused by static electricity, irregular display caused by foreign substances contained in display materials during production process such as for panel assembly, or poor display by fusion of the display surface caused by long time lighting. To resolve these problems, a method for controlling the electric property of the liquid crystal composition is exemplified. As to the abnormal lighting caused by static electricity, it was reported to control specific resistance value by adding an ionic compound such as quaternary ammonium salt to a liquid crystal composition having great specific resistance value. (For example, refer to Japanese Patent Application Laid-open Number 59-4647.) However, the ionic compound shows insufficient compatibility and solubility characteristics to conventional liquid crystal compositions. Some problems still remain. Namely, it is difficult to homogeneously disperse the ionic compound in a liquid crystal cell because the compound is easily adsorbed in the neighborhood of an injection part. Since it is necessary at the same time to control the specific resistance value and to lower the voltage-holding ratio, the consumption of electric power of the liquid crystal cell very often becomes greater.